To achieve tactical and strategic goals during military, para-military or police actions and conflicts, the destruction, or at least impairment, of targets encased within surface barriers (i.e., "hardened" targets) is often required. Since such surface barriers typically take the form of underground concrete walls or layers of soil, sand or rock, successful elimination of hardened targets often proves problematic.
Conventional bombs and rockets are largely ineffective against hardened targets. For example a conventional rocket fired from a land based location to impact on a remote target does not have the structural rigidity or the directed kinetic energy that is specifically intended to destroy hardened targets. The shape and structure of such a weapon cannot usually withstand the severe structural loads necessary to penetrate a hardened target. Nor does such a weapon include a mechanism to direct and control its kinetic energy for suitable impact to penetrate the hardened target; the needed kinetic energy from the rocket's initial firing is either too great for the rocket to survive impact or has been exhausted prior to the rocket reaching the desired destination. Moreover, the explosives in such a conventional weapon are usually set to detonate immediately upon impact which results in essentially an arbitrary release of destructive energy which is incapable of penetrating the surface barrier. As a result, various types of devices have been designed for the specific purpose of penetrating the surface barriers of such hardened targets so that the target may be exposed and eliminated. Such weapons are sometimes referred to as kinetic energy penetrating vehicles.
The existing types of kinetic energy penetrating vehicles may be broken down essentially into two groups, unthrusted and thrusted (although the particular designs within each group vary widely). Unthrusted kinetic energy penetrating vehicles reach an impact velocity due solely to the forces of gravity acting on the vehicle during descent towards the intended target. The shape and structural rigidity of the vehicle (usually a hardened stainless steel conical or cylindrical shape) combined with the kinetic energy gained from the gravitational forces promote penetration of the surface barrier of the hardened target upon impact by the vehicle. The devices then detonate an explosive payload to destroy the target. Such devices are not particularly sophisticated and therefore offer economic advantages in fabrication and production. Examples of unthrusted kinetic energy penetrating vehicles include the bombs identified as the BLU-109 and the BLU-113 by Battelle Corp and the bomb described in U.S. Pat. No. 4,488,487 to Croizer.
Thrusted vehicles on the other hand incorporate a thrusting device, usually a rocket motor, which, during descent, accelerates the vehicle to an impact velocity greater than would otherwise be achieved by gravitational forces. The increased velocity provides greater kinetic energy, which again, in combination with the shape and structural rigidity of the vehicle, enhances penetration of the surface barrier of the hardened target upon impact of the vehicle. The explosive payload therefore has a higher probability of destroying the target. Such bombs, however, must utilize materials of sufficient strength and design to withstand the added impact forces and therefore prove costly to produce. Examples of known thrusted kinetic energy penetrating bombs include the vehicles disclosed in U.S. Pat. No. 3,897,730 to Riparbelli, U.S. Pat. No. 3,935,817 to Riparbelli and U.S. Pat. No. 4,876,963 to Deffayet and the bomb disclosed in the Boosted Penetrator Study by Lockheed Missiles and Space Company, Inc. (Reference No. LMSC-F433813, 4 Dec. 1991).
Despite the proliferation of both unthrusted and thrusted kinetic energy penetrating vehicles, restraints on effectiveness persist. For example, when a target has been particularly well-reinforced (e.g., a target encased in thick steel-reinforced concrete), an unthrusted kinetic energy penetrating vehicle is often incapable of successfully penetrating the barrier. Due to the low velocity, the vehicle does not have sufficient kinetic energy to penetrate the particularly strong material.
On the other hand, a thrusted kinetic energy penetrating vehicle can be imparted with too much kinetic energy and instead of penetrating the hardened barrier upon impact, the vehicle simply disintegrates. In other words, although the thrusting device is capable of imparting sufficient kinetic energy to effectuate penetration, that level of kinetic energy exceeds the structural limitations of the vehicle.
At least one kinetic energy penetrating vehicle has been proposed that would combine certain facets of thrusted and unthrusted vehicles. In U.S. Pat. No. 5,109,774 to Deffayet, a bomb is disclosed that is unthrusted during its descent and impact with the earth but becomes thrusted after the bomb has nearly come to rest. The thrust provided after impact is provided to ensure further penetration of the device into the ground. This bomb, however, incorporates structure specifically intended to preclude excessive penetration. In addition the thrusting device only provides a limited initial impulse to the penetrating vehicle. Therefore, this device does not contemplate the need to penetrate imbedded targets let alone hardened targets. It therefore fails to solve the limitations associated with thrusted and unthrusted penetrators when considering the need to destroy hardened targets protected by surface barriers.